Commonly the target feature is a material interface whose level it is desired to monitor. The energy pulse may be of acoustic energy, or microwave electromagnetic energy. Since the wavelength of the energy utilized determines the resolution of the system, the wavelength needs to be very short compared to the distance between the transducer and the feature to be monitored.
U.S. Pat. Nos. 4,596,144; 4,831,565; 4,890,266; 4,992,998; and 5,076,751 describe methods of pulse echo measurement in which a profile of the return echo signal is digitized, stored in memory and the stored echo profile is analyzed to locate a wanted echo from the target feature. Typically such methods utilize some form of time varying threshold signal profile against which the stored echo profile is compared to enable both low amplitude distant echoes and higher amplitude closer echoes to be detected. In each case an echo due to reflection is characterized by a profile which exhibits a significant increase in amplitude relative to the threshold, providing a feature which projects significantly above the threshold curve.
In some applications, the target feature does not produce a return signal having these characteristics. For example, in sludge monitoring applications where the feature to be monitored is an interface between layers of liquid containing respectively a low concentration of suspended material and a substantially higher concentration, there may not always be a well marked echo from the interface itself, or the amplitude of the echo may be substantially attenuated in passing back through the upper layer. Instead, the interface may be characterized by a change in reflectivity from the supernatant layer, which the pulse penetrates, but in which particulate material or air bubbles reflect a certain amount of energy, and a relatively opaque layer from which relatively little energy returns towards the transducer. In such a case the interface will tend to be characterized by drop in echo amplitude beyond the interface rather than an upwardly projecting echo at the interface. In other cases also, the trailing edge of an echo will usually be characterized by an increased rate of decrease in amplitude of the return signal behind the projecting peak.
In sludge monitoring applications, as well as some other applications, the characteristic of the feature being monitored which is of most interest is not the distance of the feature from the transducer, but its distance from a bottom surface of a tank, vessel or channel. Such a bottom surface will normally provide a strong echo provided that the tank is empty, or overlying material is not too opaque to the pulse energy, and thus its position can readily be ascertained.
It should be understood that location of a target feature in pulse-echo ranging systems typically involves three functions; firstly the identification of features in the echo signal which may indicate the position of the target, secondly the selection of the feature considered most likely to represent the target, and thirdly calculation of the actual position of the target relative to the profile of the selected feature. Typically an echo return signal will consist of background which, in the early stages of the return signal is mainly due to ringing of the transducer following a transmit pulse, and then reverberation from target area and noise. Typical systems process this response sequentially according to time lapse from transmission of the pulse looking for peaks in the response projecting above the background or a time varying threshold simulating or replacing the background. A difficulty with this approach is illustrated by the example given above, when the target of interest does not necessarily produce a marked peak, although it does produce a discontinuity in the echo response.